It is known that impact printers in data processing systems use ink ribbon cartridges wherein an endless ribbon is caused to run along a print line always in the same direction, in order to ensure continuity of the printing operation and make the ribbon drive more reliable and economical.
The operation of impression members, usually wires in a printhead, causes the ribbon to wear gradually such that its service life is shortened even where re-inking devices are provided.
Especially the area of the spliced ends of the endless ribbon is affected by wear and liable to break long before the ribbon is actually spent.
As a result, the practical service life of the ribbon becomes restricted to a few millions characters, and there exists a risk of the ribbon and the impression members jamming with serious consequences for the printing apparatus.
To avoid such problems, devices have long been proposed for detecting the splice in the ribbon before it reaches the print line and discontinuing the impression operations while the splice area is facing the printing members, so as to prevent wear and damage in this area.
For instance, publication EP-A-0071726 discloses a protection device comprising a pair of pinch rollers which are entrained rotatively by the ribbon travelling therebetween.
An opening is provided in the proximities of the splice whereby, when the opening locates between the two rollers, the ribbon is no longer held in the roller nip and its entraining effect on the rollers is discontinued.
The rotation of one of the rollers can be sensed using optical sensors or sensors of some other types to provide indications of the ribbon being either in motion or at a standstill, and of the opening moving past between the rollers.
Thus, the operation of the printing apparatus can be stopped on the occurrence of a jammed condition, or the printing operations can be discontinued upon the presence being sensed of the splice before the impression members.
That device is a fairly expensive and mechanically complicated in construction to incorporate to the inked ribbon cartridge, whereas if provided externally, it involves considerable problems of installation and removal of the ribbon cartridge, and still greater design problems where the cartridge is of the type pivoted on a support.
It has also been proposed of using a conductive material arranged on the ribbon proximate to its end so as to close an electric circuit and reveal the presence of the splice.
Despite its simplicity, this solution is low in reliability because the ink not infrequently includes oily substances which are non-conductive and can form insulative coatings as may affect electric contact.